1. Field of the Invention
The present invention relates to large area, uniformly low dislocation density gallium nitride material, such as is useful in the manufacture of microelectronic and opto-electronic devices, such as light emitting diodes, laser diodes, opto-electronic sensors, opto-electronic switches, high electron mobility transistors, etc., as well as a method for making such gallium nitride material.
2. Description of the Related Art
Gallium nitride (GaN) and related III-V nitride alloys have applications in light emitting diodes (LED) and laser diodes (LD) and in electronic devices. The performance of the GaN-based device strongly depends on the crystal defects of the device layer, especially the density of threading dislocations. For blue and UV laser diodes, a dislocation density of less than 3×106 cm−2 is preferred for longer lifetime. Furthermore, GaN devices grown on native gallium nitride substrates are preferred for improved device performance and simplified design and fabrication.
Gallium nitride substrates can be prepared by various methods. Porowski et al. U.S. Pat. No. 5,637,531 discloses a method of growing bulk GaN at high nitrogen pressure. Metallic gallium is reacted with gaseous nitrogen to form gallium nitride crystals at the surface of the gallium melt. A temperature gradient is provided in the reactor vessel, resulting in supersaturation of nitrogen atoms in the cooler region of the reactor, and growth of gallium nitride crystals. The growth pressure in the Porowski et al. process is about 10 kbar and growth temperature is about 1400° C. The dislocation density of material produced by the Porowski et al. process is as low as 100 cm−2, however, the maximum size of the GaN produced by this method has been limited to about 10 mm platelets (S. Porowski and I. Grzegory, J. Cryst. Growth, Vol 178, 174 (1997), M. Bockowski, J. Cryst. Growth, Vol 246, 194 (2002)).
Hydride vapor phase epitaxy (HVPE) has been utilized to produce gallium nitride substrates. Tischler et al. discloses in U.S. Pat. No. 5,679,152 a method of producing single crystal GaN substrates by first growing a thick GaN film on a compatible sacrificial substrate and then etchably removing the sacrificial base substrate at a temperature near the growth temperature to produce the freestanding GaN substrate. Another method of separating the grown gallium nitride film from the substrate is to optically induce decomposition at the interface between the grown film and the substrate. Kelly et al. discloses in U.S. Pat. No. 6,559,075 a method for separating two material layers by using laser energy to decompose the interface layer. For GaN grown on sapphire substrates, a laser with energy larger than the bandgap of GaN, but smaller than the bandgap of sapphire, is used. When the laser shines through the sapphire substrate, the laser energy is absorbed at the GaN-sapphire interface. With sufficient laser energy density, the GaN in the interface region is decomposed into metallic gallium and gaseous nitrogen, thereby separating the GaN film from the sapphire substrate. A freestanding GaN wafer almost 2″ in diameter was produced using this method (see, for example, M. K. Kelly et al., Jpn. J. Appl. Phys. Vol. 38, pp. L217-L219, 1999). Motoki et al. discloses in U.S. Pat. No. 6,413,627 a method of making a single crystal GaN substrate material, by first growing a thick GaN film on a gallium arsenide substrate and then eliminating the substrate. The dislocation density for the typical HVPE gallium nitride is about 1×107 cm−2.
Motoki et al. in U.S. Pat. No. 6,468,347 and U.S. Published Patent Applications 2003/0080345 and 2003/0145783 describe methods to produce gallium nitride single crystal substrates with low dislocation density in certain areas but high dislocations in other areas. The high dislocation density areas are disclosed as being randomly distributed, or distributed in a predetermined pattern, e.g., in the form of periodic stripes, with the low dislocation density regions dispersed between the high dislocation density areas. In U.S. Pat. No. 6,468,347, Motoki et al. thought the GaN material produced had low dislocation density, but in U.S. Published Patent Applications 2003/0080345 and 2003/0145783, Motoki et al. clarified that material produced by the process disclosed in U.S. Pat. No. 6,468,347 had high dislocation density area randomly dispersed in the low dislocation density area. In U.S. Published Patent Application 2003/0080345, Motoki et al. disclosed methods to place the high dislocation density areas in a predetermined pattern as periodic dots. In U.S. Published Patent Application 2003/0145783, Motoki et al. disclosed methods to place the high dislocation density area in the form of periodic stripes.
The presence of high dislocation density areas on the GaN substrate necessitates precise alignment of the device structure on the low defect areas. Furthermore, the non-uniform distribution of defects may adversely affect the growth of the device layer on the GaN substrate.
Vaudo et al. U.S. Pat. No. 6,440,823 teaches the use of pitted growth to collect and annihilate dislocations, as well as high surface morphology conditions to subsequently close the pits.
Since the performance of the GaN-based laser diodes and other devices is critically dependent on the nature and extent of crystal defects in the device layer, which in turn depends on the defect structure and morphology of the GaN substrate, there is a compelling need for uniformly low dislocation density GaN substrates. Furthermore, low-cost manufacturing of GaN-based devices requires large area substrates. The prior art has failed to provide uniformly low dislocation density, large area GaN substrates.